It has long been recognized that it would be desireable to provide glazed roofs which have improved luminosity and resistance to weather and external conditions, which are relatively safe to work with and which are relatively light. While such roofs have applications for use in furnished areas, leisure spaces and covered galleries, they will be described herein by referring to a particular application : greenhouses having large vaulted ceilings.
Greenhouse cultivation is controlled, for certain types of agricultural products, by truely industrial criteria. The profitability of these cultivations depends sometimes on a narrow margin of benefit. Thus, the farmer must optimize all factors, one of the factors being the amount of sunshine.
The traditional greenhouses are generally assembled with large planar sheets of annealed glass of 4 mm of thickness. Unfortunately, the dimensions of these sheets can reach 1.12.times.1.65 m, which makes their handling difficult during assembly. Also, glass in the shape of a thin annealed sheet is relatively fragile and can only withstand a cold curving for large curvature angles, its constaint of rupture to traction being in the order of 50 N/m.sup.2. Finally, in spite of the size of the panels, the luminosity inside the greenhouses does not exceed 75% of the luminosity outside of the greenhouse. Such a reduction in luminosity is especially important in that it is known that a 1% reduction of light corresponds to a reduction of production in the order of:
1.2% for vegetables, PA1 0.9% for flowers to be cut, PA1 0.6% for ornamental plants.
It is therefore advantageous to increase the average transparency of the walls and roofs of such greenhouses by using larger volumes, which are as light as possible, while taking into account their durability, and by reducing the sizes of the elements of the framework which form opaque surfaces.
In other applications, such as in the roofing of furnished areas, leisure spaces, and covered galleries, it is important that the roofing material and the framework provide to the whole, a large luminosity which, for reasons of security, is often incompatible with traditional roofs.
Another important problem concerns the resistance of the materials to the weather. The mechanical characteristics of the covering material must be able to withstand the weather and to maintain itself in all conditions. Thus, the covering material must be able to withstand elements, such as hail, without shattering or otherwise becoming damaged.
One method which is utilized for improving the mechanical resistance of a sheet of glass is tempering of the glass. There are two types of tempering commonly employed for glass: thermal-tempering and chemical-tempering.
Treatment of tempering (thermal or chemical) of the glass establishes: on one hand, permanent tensions (forces) of compression in the external layers thereof, resulting in the glass having a superior resistance to rupture and deflection; and, on the other hand, tensions (forces) of traction in the internal layers of the piece of glass, which has for result that, in case of breakage, the piece of glass divides itself into a large number of fragments, thereby reducing the risks of injury by laceration.
Sheets of chemically-tempered glass (Chemically-tempered by a treatment of diffusion of ions) present good characteristics of fragmentation in case of rupture. However, sheets of chemically tempered glass do not present a sufficient resistance to rupture under the effect of shock of small hard objects which deeply scratch the surface of the glass. This is due, at least in part, to the fact that the thickness of the layer in compression on the surface of the chemically tempered glass is only in the order of 50 .mu.m. Thus, even slight scratches in the surface of a chemically-tempered sheet of glass can result in the scratching thereof. This fault of chemically-tempered glass is particularly important in the case of glass sheets having a large surface area, such as the windshields of automobiles.
French Pat. No. 2,138,711 proposes to remedy this inconvenience by maintaining a sheet of chemically-tempered glass in such a way that forces of compression, resulting from the flexed state, is existent in one of the sides. This side, thereby placed in compression, is then utilized as the "exterior face" of the realized article, that is to say, the face of the glass sheet which is exposed to the projection of small hard objects (for example, the exterior face of a windshield of an automobile). In this fashion, it is attempted to provide a chemically-tempered sheet of glass which has superior mechanical characteristics.
Unfortunately, the technique described in French Pat. No. 2,138,711 exclusively concerns chemically-tempered glass. This technique does not concern elastic-flexing of a thermally-tempered sheet of planar glass, in order to produce a curved sheet of glass whose convex surface is resistant to the impact of small hard objects. Finally, chemical-tempering is quite expensive, requiring a large energy input and still exposes the glass to shattering in the event that it is deeply scratched.
Because of the disadvantages described above, it is more desirable to utilize a sheet of glass which has been thermally-tempered. Thermal-tempering involves a thermal modification of the glass which strengthens it throughout its width. This means that its impact resistant qualities are not only found in the surface layers of the glass. Accordingly, even an impact which nicks the surface of the sheet of thermally-tempered glass will not necessarily result in the breakage thereof. Therefore, thermally-tempered (or hardened) glass exhibits a superior constraint of rupture over chemically-tempered glass, increasing in function corresponding to its degree of tempering, and which is able to withstand constraints of rupture of 200 N/M.sup.2 or more. Elastic-flexing of this thermally-tempered glass further increases the mechanical strength of the glass due to the creation of forces of tension and compression in the external layers of the glass described above.
Accordingly, it can be seen that there remains a need for a curved glazed roof in which the glass sheets thereof have been both thermally modified, and elastically-flexed, so as to increase the forces of tension and compression thereof and which has its curvature maintained under constraint. It can further be seen that there remains a need for a greenhouse, or the like, which has such a curved glazed roof.